Chapter 4: Fundamentals of Adhesion

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2 Learning Objectives To understand the principles of adhesionTo understand the relevance of adhesion and adhesives to natural products

3 Relevance of Topic to Renewable Materials70% of woods applications require some gluingFibers from agriculture crops need to be assembled into some structure requiring adhesionWood is bonded in over 70% of its applications (Hemingway 1989)Diminishing high quality wood resourceStraight, clear structural timber from sawingIncreased use in compositesUse of agricultural fibers that inherently start as smaller constituents than woodMove to adding value to agricultural residuesComposite from biological sources have been used since ancient Egypt and China

4 Importance of AdhesionIn order for two or more materials to perform as one material, as a composite, there needs to be an adhesive bond between these materials that allows them to deform as one. This is called continuity of strains. The adhesive, the adhesive bond, and the materials must be able to withstand external stresses and strains to perform as a composite.

6 Overview of Topics Definitions of concepts related to adhesionAdhesion mechanisms and theoriesThermoset adhesives for cellulosicsSurface preparationSeizing/coupling agents for fibers and fillers in composites (thermoplastic or thermoset)

7 Key TerminologyAdhesion – the tendency for the surfaces’s dissimilar materials to cling togetherCohesion – molecular attraction by which the particles of similar bodies are united throughout the massInterface – the surface forming the common boundary between two materials in contact (2D)Interphase – the volume around the interface that possesses properties unique from the joining materials

9 Classes of Adhesives Used for Bonding Natural Fibers/MaterialsPolyvinyl AcetatesFormaldehyde basedUreaformaldehyde, phenolics, resorcinolMelamineTannin basedProtein based (casein, blood, soy)Isocyanate basedPMDI, urethanes, ureasOthersEpoxies, acrylics, hot-melt, starchThis is a list of some of the most common wood adhesives, but it is not exhaustive or all inclusive. Many of these adhesive bond to natural surfaces using a variety of chemical and physical mechanisms.

10 Adhesives UsesDifferent adhesives types have varying end uses. The uses vary depending on adhesive properties, such as strength, toughness, and durability, and cost. Many of these properties depend on the bonding mechanism and the associated chemistry.From: The Woodhandbook (http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch09.pdf)

13 Chemical BondingFormation of covalent bonds between adhesive and adherentFormation of strong environmentally stable bondsWater proofConsume hydroxylsBond(kJ/mol) C-C 348 C-N 293 C-O 358 C-F 485 C=C 614 Hydrogen 1-5There are several chemical bonds that are important to bonding wood that will impact the strength and durability of adhesion. Ideally, one tries to covalently link a material to the wood surface by forming any of the above bonds. However, hydrogen bonding is the most common binding mechanism. As one can observe, many hydrogen bonds must be formed to produce the equivalent strength to a covalent bond. Also, hydrogen bonds are not durable in the presence of moisture.

14 Strength of Adhesive BondsPrimary bonds are very strong, but they are not the most important bonds in wood adhesion. They have not been observed to exist either because they don’t exist or because they are present in such small quantity that they are beyond detection (Johns. The Chemical Bonding of Wood. In Wood Adhesives: Chemistry and Technology. Vol. 2. A. Pizzi Ed. Marcel Dekker. New York. 1989).Source: Pizzi. Advanced Wood Adhesives Technology. Marcel Dekker. New York

15 Example of Chemical BondingWood and isocyanate have the potential to form covalent chemical bonds. However, the pathway for the formation of polyureas is much more likely given the abundance of moisture present in wood. It is also thermodynamically preferred. To date, an isocyanate and wood chemical bond has not been discovered under realistic processing conditions.Harper et al. 2001

16 Diffusion TheoryThe entanglement of polymer chains in solution or meltPolymer viscoelasticityt characteristic relaxation time, t0 is a small time step, N is the number of repeating units, a is an exponent (~ )t > t the polymer chains are “frozen” or glassyt < t the polymer chains flowThink of a bowl of spaghetti where the noodles become entangled. This could also be thought of as velcro, polymer chains may be hooked into the loops of other materials.

18 Block Co-polymers DiblockHeptablockPentablockTriblockDiblockOne strategy is to produce a molecule with different blocks along the backbone that are similar to each of the surfaces that one is trying to adhere. These ends can diffuse into the surface of one material adsorb on to that surface.*Eastwood, E. A. and M. D. Dadmun (2002). Macromolecules 35:

19 Surface density of “adsorbed layers”Polymers assemble on surfaces out of solution or melt to a lower energy stateDensity at the surface depends on the ergodicity of the spaceTails, loops are createdSites for “coupling” the matrixIdeal to have covalent bonds “chemisorption”Optimal “sticker” densityB. O’Shaughnessy 2003

20 Lifshitz-van der Waals InteractionWe must consider the thermodynamic state of the surface. Thishas an impact on the wetting or spreading of an adhesive on thesurface of a fiber.g = surface tensionG = Gibbs free energyA = interface areaEquilibrium spreading pressureDispersive forces have a critical role in the wetting, thus spread, of adhesive on a surface. This is important to assure intimate contact between the adhesive and adherent.Wa = work of adhesionqe = equilibrium contact angleS = surface V = volume L = liquid

21 Lifshitz-van der Waals Interaction (cont.)This illustrates the angle that a drop of liquid makes on a surface. Often, this angle changes with time and is asymmetric for natural materials. In example, a drop of water will wet along the grain of wood faster than across the grain.L.-H Lee 1991If glv < gsv and q < 45 then good wetting is achieved.

23 Examples of Wetting and Surface PropertiesFigure 1: A water droplet on a freshly sanded tangential surface of ACQ treated Southern pine.Figure 2: A water droplet on a freshly sanded tangential face of acetylated wood.

27 Electrostatic InteractionsBuildup of a charge on a surface caused by contact with other surfacesThis is noticeable when own surface is highly resistiveMetals in contactTransfer of e- across the interface (electrical double layer)Creates a force of attractionCan occur across some polymer metal interfacesIon – ion interactionsLittle relevance to bio-based fibers and polymers since most are poor conductors and insulating materialsAlthough weak, the attractive force between a proton and electron is 40 times greater than that of gravity

28 Examples of Electrostatic InteractionsPlastic packaging on handsPaper on CDsPaints and coatings for metals

29 Adhesives Used for Natural Fiber CompositesThermosetHeat causes polymerization and cross-linking of the adhesivePMDI, UF, MUF, MF, PF, acrylics, epoxiesThermoplasticHeat is applied to either melt (semi-crystalline) or raise the temperature above the glass transition temperature (amorphous) to allow a polymer to flowEVA, MAPPSolvent lossA colloid or polymer is dispersed or dissolved in a solvent and the solvent evaporates away leaving a solid filmEthylene vinyl acetate (PVA) – latex based adhesiveOthersReactive systems (two part or other) – cyanoacrylates, epoxies, polyestersRadiation cure – acrylics and epoxies

31 Urea-FormaldehydeAn amino resin that is the polymeric condensation product of a reaction with formaldehyde and urea.Most panel boards worldwide are made with UFAdvantages:Water solubility prior to cure, hard, flame resistant, clear, good thermal stability, can be tailored to a wide range of curing conditions, inexpensive compared to other resinsDisadvantages:Bond durability – caused by hydrolysis of the aminomethylenic bondEmits formaldehydeWax is usually added to wood products to increase water resistance of the end productThese resin have been used for many years with great success in interior applications at a low cost and high volume. However, new regulations are causing wood manufacturers to look for alternatives because of their high formaldehyde off-gassing levels.

32 Using UF Applied by air atomization in a blender or blow-lineViscosity range = 30 – 300 centipoiseReaction products and cure temperatures can be controlled by pHIn general these are acid catalyzed at a pH ~4.2 (Maloney 1993)Curing temperatures 100 – 190°CCure time is dependent on many factors including furnish moisture, pH, molecular weight, free urea, and temperature. All of these factors can be controlled by the manufacturer.

33 Melamine-FormaldehydeInclude MF (melamine-formaldehyde) and MUF (melamine-urea-formaldehyde)Similar to UF, MF is formed by a condensation of melamine to formaldehyde. The amino group in melamine reacts completely with formaldehyde groups leading to complete methylolation. Up to six formaldehyde molecules may be attached (see Pizzi 1994).AdvantagesMore durable than UF, lower formaldehyde emissions, high tack with low viscosity (important for fiberboard), cure over a wide range of pHDisadvantagesMore expensive than UF, less durable than phenol formaldehyde

34 Phenol FormaldehydePolycondensation product of phenol and formaldehydeFirst commercial polymer (bake-lite) and still of large commercial importanceUsed in structural and external panels (OSB, Plywood, Parallam)AdvantagesNon-conductive, heat resistant, water resistance, moderately inexpensiveDisadvantagesFormaldehyde, brittle, distinctive redish-brown color, much more expensive than UF, need a higher temperature and longer cure time than UF or MUF

35 Resorcinol ResinsResorcinol resins may be a combination of resorcinol and PF resins. They are two-part systems that are mixed with a catalyst to cure at room temperature. They are primarily used in laminated beams, finger joints, and structural applications.AdvantagesVery resistant to moisture, strong bonds, long-term durabilityDisadvantagesCan have long curing times, expensive, reddish-brown color

36 IsocyanatesPrimary reaction is isocyanate and water to form an amine and subsequently a poly ureaUsed in structural, exterior panels that are strong and moisture resistantAdvantages100% solids, no formaldehyde, wets wood better than PF, does not introduce excess moisture, durable and strong bonds, foamsDisadvantagesMuch more expensive than formaldehyde based adhesives, sensitizing agent, foams, bonds metal

38 EpoxidesMany different chemicals that consist of an epoxide ring that reacts with an amine or free radical to cure with time, heat, or ionizing radiation. Usually, epoxies are cured as two part systems with a resin and a hardener.AdvantagesGood adhesion to a wide range of materials depending on formulation, a wide range of formulations are available, reacts completely with little to no VOC emissions, moisture resistantDisadvantagesNot a lot of information of the efficacy and durability of wood bonds, some components of common epoxies are carcinogens (bisphenol-A), expensiveCurrent uses include repairing glulams, molded wooden boats, bonding wood to other materialsImage from:

39 AcrylicsMany different chemicals that consist of an acrylate or methacrylate group that reacts with a hardener or free radical to cure with time, heat, or ionizing radiation.AdvantagesGood adhesion to a wide range of materials depending on formulation, a wide range of formulations are available, moisture resistant, heat resistant, methacrylates make a stiffer, but often harder to cure bond and are more expensiveDisadvantagesNot a lot of information of the efficacy and durability of wood bonds, may off-gas harmful vapors, expensiveCurrently, acrylics have very limited use in wood, but there has been a lot working on impregnating wood for flooring, counter tops and other application. There is a large potential for its utility in radiation cure in wood products.

40 Protein BasedMade from proteins from animals, blood, casein (milk), and soyUsually mixed with water and lime and cure at room temperature most commonlyNew soy adhesives are being combined with formaldehyde based adhesives to reduce VOC’s and use a renewable feedstockAdvantagesHigh dry strength, good thermal resistivityDisadvantagesPoor moisture and biological resistanceStill used in interior doors and furniture because of good fire performance

41 MAPPMaleic anhydride polypropylene is an adhesion promoter used in wood-plastic composites. It has shown a great deal of efficacy in promoting better adhesion between wood (also other natural fibers) and polyolefins.

43 Poly (vinyl) Acetate (PVA)Solvent loss system that is usually water based that can be cured at room temperature under pressure. Solvent loss systems need intimate contact by applying pressure to form adhesive bonds.AdvantagesCheap, high dry strength, non-toxic (can be used in food contact applications), can be combined with cross-linking agents and catalysts to increase durability, dries clear to varying colorDisadvantagesLow moisture resistance (cross-linking improves this, but makes it more toxic and expensive), low heat resistanceUsed in many furniture, molding, doors and architectural applications. It is commonly referred to as carpenters or wood glue